“Ever since Bruce purchased a ($7 million-plus) parcel of land down there, he and ex-wife Demi have been going to the romantic island on their private jet to oversee contractors who are busy building a love pad they’ve designed.

“Sometimes Demi comes only with her boyfriend. Other times Bruce shows up with his flavor-of-the-day. And then there are times when they all come together!”

During her recent trip to the tropical getaway with Oliver and her three daughters — Rumer, 13, Scout, 10, and Tallulah Belle, 8 — Demi managed to get away from all four to link up with Bruce, who was vacationing there with Estella Warren.

“Bruce and Demi went shopping together at a local pharmacy,” revealed the insider. “If I didn’t know better, I would have thought they were lovers. They were holding hands, caressing each other and giggling like newlyweds.

“While Demi was trying on sunglasses, Bruce stood behind her with his chin resting on her shoulder, and watched her in the mirror. She tried on at least a dozen pairs until she found one that Bruce liked.

“Before they lift the store, he gave her a small peck on the cheek and they walked out holding hands again.

“I wonder what Oliver and Estella would have said about that!”

Bruce and Demi are perfectly comfortable with the unconventional arrangement — and it’s really nothing new to them, revealed a source.

“Bruce has a house right across the street from the Hailey, Idaho, home where Demi and the children live with Oliver.”

Soon the uncouple will enjoy close quarters in the Carribbean as well, added the insider.

“Construction of the large home and two adjacent guest houses on the seven-and-a-half-acre property is still underway, but by the looks of things, the main house where Bruce and Demi will be staying is going to be a sprawling mansion.

“When they aren’t using their estate, they plan to rent it for a small fee of $13,000 a day!”

Concluded a buddy of the two stars: “Bruce and Demi’s frolicking foursome situation at their new island hedeaway may seem weird to many people, but to them it’s perfectly natural.

“They’ve remained very close to one another, mostly for the sake of their daughters. They’re still the best of friends — and very affectionate.”